Log skidders used in forestry operations have a grapple mode in which a log grapple mechanism comprising a boom and grapple combination can be mounted on the rear of the skidder. The boom may be fixed with the grapple mechanism modified so it may be lowered to the ground or the boom may be hydraulically movable to lower the grapple to the ground. In either event, the grapple mechanism comprises a pair of reciprocally movable tongs which may be actuated by a first hydraulic cylinder or cylinders to open and close around a group of logs. Typically, in operation the grapple boom is lowered by means of a second hydraulic cylinder or cylinders with the grapple tongs open to engage a group of logs, then hydraulic pressure is applied to the cylinder or cylinders actuating the grapple mechanism to close the tongs about the group of logs. The boom or the grapple is then raised to lift a forward end of the logs off the ground. Then, the logs are dragged by the skidder to another location for loading or unloading.
One of the problems in the above sequence of events is that once the logs are gripped within the grapple tongs and are being dragged by the skidder there is a possibility that the logs will shift to create a smaller mass about which the grapple tongs are closed. Such shifting, if it is not corrected by a mechanism to maintain constant pressure on the logs, would cause the grapple tongs to open enough to permit the loss of a portion or even all of the load of logs requiring the operator to begin the operation of the grapple mechanism all over again and resulting in wasted effort and much inefficiency.
Accordingly, a number of solutions have been offered to the problem. In some cases a simple check valve has been disposed between the source of hydraulic fluid and the cylinder to maintain pressure in the lines. In other cases a hydraulic connection has been provided between the hydraulic cylinder operating the grapple and the hydraulic cylinder operating the boom to equalize pressure in both boom and grapple. A third form of solution is a pressure equalization circuit in which an excess pressure built up in the grapple actuates a pilot valve to dump excess fluid when the pressure is too high and increase fluid flow when the pressure is too low. All of these proposed solutions have shortcomings and thus the search for an ideal grip control circuit continues. The present circuit follows along the lines of the third alternative but offers a solution that is unique when compared to the available prior art.